My husband, who says himself that he has a very low pain threshold, had a colonoscopy and said afterwards that it was not at all distressing - he could hardly remember it. He must have had a lot of sedation because it was quite a while before they would discharge him to come home. All the other people that I know who have had a colonoscopy have said it was fine - not at all painful.
Though I had sedation, I really didn't feel as if I had. Like most people, I've had sedation in the past and felt rather "out of it" but this time I felt completely with it. I believe that is the reason I found it a very unpleasant experience. I think the poster who said that the idea is to give as little sedation as possible and get the patient out as quickly as possible has probably hit the nail on the head.
Given that you have had distressing experiences before, I think you should discuss it with your GP and discuss whether it would be possible to have a GA (which I think they will be very reluctant to do) or high sedation. It is definitely worth speaking to your doctor as this must be causing you anxiety and stress.